Gases are compressed in many different types of industrial facilities and for a variety of purposes. For example, air is compressed, cooled and introduced into one or more distillation columns in an air separation plant. In a liquefier, a gas is compressed and sufficiently cooled to a liquid. There are many other examples of industrial facilities in which gases are compressed.
In any facility, although a single compressor stage can be used to compress the gas, more typically, the gas is compressed in multiple, sequential compressor stages. The reason for this is as the gas is compressed, its temperature rises. The elevated gas temperature requires an increase in power to compress the gas. In a typical compressor installation utilizing individual stages, each stage uses a centrifugal compressor in which gases entering an inlet to the compressor are distributed to a vaned compressor wheel that rotates to accelerate the gas and thereby impart the energy of rotation to the gas. This increase in energy is accompanied by an increase in velocity and a pressure rise. The pressure is recovered in a vaned or vaneless diffuser that surrounds the compressor wheel and functions to decrease the velocity of the gas and thereby increase the gas pressure of the compressed gas. The compressed gas is discharged by a volute of spiral-like configuration that terminates in an outlet of the compressor. The gas is therefore discharged from the outlet at right angles to the incoming gas to be compressed.
Since the gas has been compressed, its temperature has also increased. The heated compressed gas is cooled between the compressor stages by intercoolers in which the heated compressed gas cools through indirect heat exchange with a coolant, for instance, air or water.
Typically, the multistage compressor installation described above is driven by a common gearbox having an electric motor driving a bull gear that in turn drives pinions that are connected to compressor shafts that rotate the compressor wheels. Since the gas is gradually compressed from stage to stage, each compressor pinion may rotate at a different speed and torque related to the pressure rise to be accomplished in a particular stage on that pinion. This arrangement is particularly advantageous in an air separation plant in which it is desirable for a common plant design to be utilized with different gearing arrangements that can be used to meet production requirements for a particular plant. Additionally, since an air separation plant requires refrigeration that is generated by turboexpanders, the work of expansion can easily be recovered by gearing between the turboexpander and the bull gear. The disadvantage of such arrangement is that since the compression stages are arranged around the bull gear, the piping or conduits connecting the stages to the intercoolers can become quite convoluted. Each bend in the conduits results in a pressure drop due to turbulence induced in the flow by the change in direction of the gas within the bend. Additionally, the conduits lead directly to the intercoolers, a rapid increase in flow area results at the connection of the conduits to the intercooler. This rapid increase in flow area also results in a pressure drop due to the resulting turbulence that is induced into the flow. Also, mal-distributions in flow can occur in the intercooler such that not all of the heat exchange passages are utilized effectively.
Another disadvantage of the arrangement discussed above is that there are irreversible heat loses in gearboxes used in housing the bull gear and pinions. Further, since the torque is transmitted in a geared arrangement, there are certain mechanical requirements for the size of gear teeth resulting in limitations in the size of the pinions and therefore, speed that can be induced in each of the compression stages. These limitations are overcome through utilization of variable speed electric motors driving each of the compression stages individually. An example of this is shown in US Patent Application No. 2007/0189905 that is specifically designed to overcome the limitations discussed directly above. However, in this patent application, there is no appreciation of the pressure drops that can be induced due to the connection of the intercoolers with the compression stages.
As will be discussed, the present invention provides a compressor installation utilizing centrifugal compressors and interstage cooling in which each of the compressors is independently driven and positioned in a manner that incorporates low pressure drop connections between the stages and to the intercoolers located between stages.